Dredging is an excavation activity usually carried out underwater, in shallow seas or freshwater areas with the purpose of gathering up bottom sediments and disposing of them at a different location. This technique is often used to keep waterways navigable. It is also used as a way to replenish sand on some public beaches, where sand has been lost because of coastal erosion. While the intentions of dreging can be good, it can also have negative environmental impacts such as disturbance to aquatic ecosystems and dredge spoils containing toxic chemicals that can have an adverse effect on the disposal area.